text:
How does the vertical height of the wings on a plane affect flight relative to its position? E.g., how does a higher high wing affect flight?
I think I understand how a high wing differentiates from a mid wing or a low wing; what I am asking is how, for instance, a very high high-wing makes itself unique in flight from a very low high-wing.
So let's say you have a high-wing aircraft. Raise the wing vertically to make the high-wing even higher. What does this do to the airplane's flight characteristics?

An important factor is the "pendulum effect" or "pendulum stability"-- google ASE questions or answers involving these terms-- I'll post some links when I get a chance--

Highly related -- https://aviation.stackexchange.com/questions/53437/does-pendulum-effect-apply-to-hang-gliders-or-any-aircraft6

Re link above-- and see my answer https://aviation.stackexchange.com/questions/53437/does-pendulum-effect-apply-to-hang-gliders-or-any-aircraft/56548#56548  .  Could be further improved by incorporating some of the specific points I've made in comments to two of the answers to the present question --

Another highly related question (arguably a near-duplicate, except the current one is broader)-- https://aviation.stackexchange.com/questions/26396/why-are-high-wing-aircraft-more-stable -- this is a topic that's gotten quite a lot of exposure on this site, and you can see that there's a bit of an ongoing controversy, with several contributors consistently falling into one "camp" or the other--

Another highly-related question https://aviation.stackexchange.com/questions/64380/contribution-of-wing-vertical-location-to-roll-stability

First off, it's important to realize that a wing does not constitute a fulcrum in a pendulum. There's no magical hinge that a plane rotates about, hidden in the wing. This is known as the pendulum fallacy.
Instead, think of an airplane purely in terms of incoming airflow. If the air is coming from straight ahead, there's not much difference between a low wing and a high wing. The main difference is that interference effects at the wing root are most prominent at the suction side (top of the wing) in a low wing aircraft, which may affect performance. Mid fuselage would be ideal, like on most gliders.
A more prominent effect is when the wind is not from straight ahead but from the side, like in a gust or sideslip. This will increase the pressure at the windward side of the fuselage. For a low wing, this high pressure pushes the windward wing down, whereas a high wing will be pushed up. Ideally, you want the latter, since the induced roll will make the plane slip the other way, stabilizing towards a coordinated turn. So a high wing is superior in this regard, although dihedral can also provide this stabilization. A high wing far away from the fuselage would lose this benefit.
In reality, wing placement is governed mostly by practical issues like obstacle or water clearance, landing gear and engine placement, and not having a wing spar in the middle of the passenger cabin. Dihedral or anhedral is then used to achieve the desired stability.

Consider an unswept flat wing (no dihedral), with a streamlined heavy dense weight mounted far below the wing, connected to the wing by an infinitely thin (but completely rigid) pylon.  Interference effects between pylon and wing are nil.  Also the weight is so compact that sideforce effects generated by the impact of sideways airflow against weight are nil.  If any sideways airflow is present, won't the simple fact that the wing's drag vector now has a sideways component, acting above the CG of the whole system, create a roll torque in the downwind direction (i.e. a dihedral-like effect)

(The problem should be able to be analyzed either by treating wing and "pendulum" weight as a single rigid system, or by treating them as two separate bodies and analyzing what torque one exerts on the other -- with the same answer yielded by both approaches-- )

(Potentially could (and may) refine this into a new question after some further thought and input -- )

(One issue is I'm a little confused about whether (or why) we would still see the same roll torque if we analyzed the problem from the point of view of the wind frame of reference rather than the aircraft's body frame of reference.  On the other hand, if the roll torque is generated by actual dihedral, or by sweep, then the answer to that is obvious.)

Considering the Catalina as a giant hang glider shifting its weight... the quandary becomes:  will it roll faster than it accelerates sideways, just like balancing a broomstick.  If it is "bottom heavy", the weight is dragged behind the wing, if it is top heavy, the weight (unstably) falls forward, acceleration (from the bottom) must match its rate of (angular) topple.  Interestingly, aerodynamic drag helps stabilize  top-heavy, and promotes bottom heavy roll, but once acceleration goes to 0 drag alone must stabilize top heavy.

Proper analysis seems to require center (and magnitude) of horizontal lift, drag, and center of mass.

@quietflyer Yes, I must admit I neglected skin drag of the wing. What you're describing is similar to adding a lot of dihedral. So it will tend to coordinated flight, but sadly a graveyard spiral is also coordinated flight...

@RobertDiGiovanni I'm sorry but once more I have no idea what you're saying. Could you please try and formulate concrete questions or suggestions, or otherwise take it to the chat room?

@Sanchises -- and now consider that we take said wing as described above, and add tip plates that project equally far above and below the wing.  In a sideslip (sideways flow) they will generate sideforce.   Without the heavy mass rigidly mounted below the wing, this will generate no roll torque, because they are shaped so that they generate no dihedral or anhedral effect, projecting equally far above and below.  But with CG of the system fixed far below the wing, this sideforce will create a powerful dihedral-like roll torque, potentially far more effective than just skin drag alone.

(OK, because of wingtip vortices etc, the tip plate surfaces perhaps should not be exactly symmetrical above and below the wing to achieve zero roll torque in the case where CG is up by wing, if so modify shape as needed, the point of the thought experiment should still be clear-- )

I think this kind of thought experiment opens the door to understanding what is going on with paragliders, where the wing itself actually has a very distinct anhedral geometry, yet all that side area acting high above the CG of the whole system (treating the suspension lines as rigid struts and treating the wing fabric as a rigid structure) generates a powerful dihedral-like roll torque in the presence of any sideslip at all.  Again we could expand the thought experiment to consider effects of flexible structure but that's just an additional complexity or level of detail.

Similarly, in case of hang gliders rather than paragliders, mounting pilot's single suspension strap to a point well below the CG of the wing itself, as per some early designs, surely creates some "pendulum stability" due to effects noted above, including fact that "billowed" shape of wing exposes considerable side area to any sideways component in flow--  (all this dealing with the hands-free case and keeping in mind that when pilot is hands-free, his mass essentially acts as if is concentrated right at the point where the flexible "hang strap" connects to the rigid frame of the aircraft...

(Said way too much, should open a chat room and move comments to there I guess-- but I think these points are worth some consideration-- )

All these effects definitely are based on effects of sideways flow, not fully coordinated flight-- but a turn generally does involve some sideslip unless we are making specific efforts to prevent it (i.e. w/ rudder) --

@Sanchises sorry about all the clutter.  I would start with the broomstick balanced by hand. Just draw in the vectors.  Interesting qf point about coordinated flight, and how off center drag would affect it!  Is a hang glider "coordinated", or does it slipturn (and still "center the ball")?

@quietflyer we have fun with "pendulum stability", but drag/lift vectors around the center of mass hold well with the modeling. Stuff basically weathervanes to the relative wind.

So let's say you have a high-wing aircraft

Allright, I choose the Dornier Do-24 ATT, a very well behaved high wing airplane with excellent seaworthiness. Below I show it in frontal and side view with lift and weight forces symbolized by arrows. I decomposed the lift into its zero-lift moment and put the lift vector at the neutral point, where all angle-of-attack-dependent lift acts, so the lengthwise distance between weight and lift vector is proportional to the static longitudinal stability and is delineated by a pair of arrows. Of course, when you shift the lift vector into the center of pressure, it will be aligned with the weight vector and the zero-lift moment will disappear.

This is my baseline high wing design, and next I raise wing and tail by a grotesque amount. No less will do to make the differences obvious.

Raise the wing vertically to make the high-wing even higher. What does this do to the airplane's flight characteristics?

If we neglect the additional vertical surface at the back, what has changed are inertias in all axes, which have grown by the increased distance between the horizontal surfaces and the fuselage. Also, the center of gravity has moved up a bit.
What are the consequences for the flight characteristics?

Maneuvering will be more sluggish and will require larger control surface deflections.
Rolling will cause a sideways shift at the pilot's station which might be a bit confusing, but one can get used to this.
Stability hasn't changed. Static stability is identical to the baseline and since the tail lever arm is still the same, pitch damping is also the same. The increased fore-back motion of the wing during a pitch oscillation will mostly make itself felt in inertial changes, aerodynamically this is of minor consequence.
The higher engine placement means higher pitching moment contributions from the engines. More downforce on the horizontal tail is needed to compensate the pitch-down moment from the high engines, and throttle changes will require high pitch trim changes.

Now what about lateral stability? And what about pitch changes away from level flight at cruise speed? For this we need another sketch. The baseline first:

On the left, the airplane flies a coordinated turn with a bank angle of 45° and on the right it is in level flight close to stall at 15° angle of attack. Turning adds a centrifugal force which acts at the center of gravity and needs a bank angle and lift increase so lift is sufficient to balance the resulting mass forces (denoted as R here). Since both forces  act along the centerline, no imbalance or instability comes with the high wing arrangement. However, the side sketch now reveals a larger distance between the mass and lift forces, which means that the airplane becomes longitudinally more stable at low speed. The zero-lift moment has to become larger to trim this high pitch angle by incasing the negative elevator deflection. Note that the green circle has grown in size and weight to reflect this. The increased elevator travel helps to keep stick forces at low dynamic pressure up and requires more elevator travel for stalling than in a low wing configuration.
And now we do the same for the high wing version:

Again, not much has changed, only that the stabilizing effect of a high pitch attitude is now even more pronounced and the zero-lift moment needs to become even larger than before. This will require a bigger tail, or only a small range of angles of attack can be trimmed. Turning feels the same, apart from the need to overcome the higher inertia with more forceful commands, and again no instability can be seen.
But a low wing ... a low wing will still show the same results. Angle of attack changes mean less change in static longitudinal stability and lateral stability is unaffected. Only the effect of the fuselage on the yaw-induced rolling moment will require a low wing to have more dihedral which makes it ever so slightly less efficient. But mayhem and carnage fail to manifest themselves.
And what does an uncoordinated turn do? Now the airplane will sideslip and the weight vector will not be aligned with the centerline in the frontal view. But the lift vector, still orthogonal to the wing, will, so no rolling moment from weight or wing lift develops. Only the side force of the vertical tail and the fuselage might cause a very small roll contribution which can easily be balanced with a bit of aileron. Again, mayhem and carnage fail to manifest themselves.
The central fallacy here is the comparison with the broom, balanced on a fingertip. Airplanes (and drones or rockets, for that measure) are different. Conservation of momentum dictates that all rotations take place around the center of gravity, and lift, being the result of surface pressures, is always orthogonal to the surface of the wing. In consequence, regardless of wing position, a bank angle will not cause a destabilizing rolling moment.

+1 Ir's a good answer. Would that still require all the comments on other people's answers?

@Koyovis Tried to nudge them to the truth but the force is strong with the pendulum fallacy ...

See my recent comments under Sanchises's answer about roll torque generated by wing's drag vector during a sideslip, acting well above the CG -- -- also the rolling effect of any sideforce generated by the wing due to wingtip endplates, etc, or even due to side area exposed by dihedral, gull-wing geometry (dihedral inboard, anhedral or flat outboard), or even anhedral geometry-- the paraglider case is particularly instructive because during sideslip it manages to generate a net dihedral-like roll torque with an exteme anhedral wing geometry-- all due to wing being so very high above CG--

-- all due to wing being so very high above the CG of the whole system with so very much side area exposed to the sideways wind component during sideslip --

Will work on merging those (and these) comments into a new answer and then deleting them -- !! –

There are good arguments for high and low wing stability while turning.  With a high wing, if one relaxes the elevator before rolling out, the sink will help it roll upright.  With a very low wing/very high CG, relaxing elevator first may result in an inversion.  Staying with that broomstick for now.

You can imagine that any change to the outside shape of an aircraft or anything that affects its centre of gravity or angular inertia will affect its flight characteristics.  On the face of it, a high wing will impart more stability but it may compromise other things such as placement of engines and undercarriage, airflow over the empennage and control surfaces, characteristics while in ground effect and so on.  In aircraft design there are very many factors to take into account and it’s difficult to consider one in isolation from the others.  Trivially, high wing has more inherent stability than low wing but there are many ther factors.

Please explain why a high wing has more stability, and why this should be trivial. You don't mean the pendulum effect fallacy, do you?

@Peter Kämpf forgive me I’m primarily a paraglider pilot and rely heavily on this phenomenon as I have no empennage.

So while the pendulum effect may be regarded as a fallacy, a claim supported by the fact that most rockets have the engines at the bottom, surely it’s intuitive that an aircraft with a large weight positioned high above it on a tower would have rather poor flight characteristics, no?

Placing the wing above the center of mass of the airframe means that perturbations in the three axes will generate a righting moment which tends to oppose the perturbation. The plane can then be positively stable in the hands-off condition and will to some extent "fly itself".
Placing the wing (the center of support) below the center of mass is then like balancing a pencil (with its center of mass at its midpoint) by its tip on your finger (the center of support). Once the pencil begins to rotate and fall, the rolling moment becomes stronger (as the center of mass gets offset relative to the center of support) which means those perturbations will grow instead, and the plane will be divergent (dynamically unstable, i.e. it will display "negative stability") in all axes- and if flown hands-off, it will try very hard to invert itself and fly upside-down. Mayhem then ensues, and carnage will result.

While a lower wing is indeed less stable in pitch, you can't be serious about the rest of your answer, can you?

@PeterKämpf, yes I am unless I misunderstand the question- will edit. Please review it :-)

@PeterKämpf, the fuel tank atop the bomber is empty and weighs much less than the bomber and I would assume a skilled pilot or an autopilot could keep the center of lift centered under the center of mass so mayhem and carnage are avoided, yes?

@nielsnielsen very old, but good discussion here about the "balancing pencil" with aircraft.  Rolling to the side will introduce a lateral acceleration, which helps rebalance.  Additionally, drag from the lateral motion (on the pencil) also resists its toppling.  A little top-heavy helps manuvering.  The 747/Shuttle combination did have banking limits for reasons mentioned, but again, aerodynamic effects are strongly in play.  Generally, a banking plane will sink, top heavy (and bottom draggy) will tend to invert as a badminton shuttlecock.

@nielsnielsen The "fuel tank" held a rocket which could weigh up to 50 tons - not trivial, and not a problem for flight stability. Higher inertias meant a more sluggish reaction to control inputs, but no loss of stability. I have now added my own answer, let me know if it helps in understanding the issue.

Weight distribution is critical in determining the effects of making a high wing even higher.
While trial and error with balsa models will keep one busy, some educational background is very helpful in eliminating the more embarrassing moments.
As a designer, try to plot any changes in center of gravity and center of drag.
Generally with aircraft, raising the wing moves the center of drag higher. This can make the plane more difficult to control in a crosswind. As seen with the PBY 3 Catalina, the designers raised the center of gravity by placing the engines on the wings to reduce this effect$^1$.
Having the Cdrag above the Cg means that ailerons must be held in a turn or the plane will tend to roll away.  This is the "self righting tendency".
Again depending on the new CG, raising the wing may also give the plane a greater tendency to pitch up.  As this will vary with forward airspeed and AoA, one might wonder if too much of this may actually be undesirable.
$^1$ keeping the props away from the water as well


text:
Number of US Commercial Aircraft versus Rest of the World
I have been trying to obtain information on the number of commercial passenger aircraft (i.e. jetliners) that US airline companies have compared with the number airlines for the whole rest of the world have. That is, if the US has x number of commercial passenger aircraft, how many do all other countries combined have (is it 5x, 10x, ...)?
I would think that the rest of the world has perhaps 5-10 times more commercial passenger planes than US airlines do, but I can't find information about this on the FAA website.
Any references/guidance is appreciated.

Try doing some research beyond just the FAA (which is responsible for only one of the 120+ countries in the world). The nerds who contribute to Wikipedia have created pages including fleet size information for most of the airlines in existence. Find all the airlines, sum their fleets by country, and you should at least have an order-of-magnitude sense of what the answer is.

There is a convenient WP list of commercial jet airliners with numbers in service, though I believe this also includes cargo variants. It totals about 27,700 aircraft as of July 2020.
The US Bureau of Transport Statistics collect data from US airlines; one of the datasets they publish is Form 41 Financial Data, where "large air carriers" (revenues over \$20m) give general financial reports each year. Schedule B-43 lists an inventory of aircraft in those airlines. We can probably assume that not many airlines with under \$20m revenues operate a significant number of jet airliners.
A very quick and dirty count finds that as of 2020, US airlines reported 7117 operational aircraft, 4029 of which had a capacity of >100 seats and 5249 had >50 seats. All of these except 32x 70-seat Dash 8s seem to be jets, so let's take that as a cut-off for "jet airliners" - 5237 of them.
This suggests that US airlines have approximately 20% of the world's jet airliners, right at the higher end of your estimate. The number is probably variable by a few percent either way depending on how you account for cargo aircraft in the totals, but even a generous estimate here would suggest the US airlines do not go above about 25%.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
What is the operating altitude of jetliners?
I am performing a science project, and this involves knowing the average operating altitude at which commercial passenger airplanes (jetliners) fly.
I am trying to find minimum operating altitudes and maximum operating altitudes of commercial airliners so that I can estimate an average operating altitude. Unfortunately, I can't find any documentation/papers/articles on the FAA website which give information on this. All I can find are blog articles and other informal articles, but I need something more concrete and formal to cite. Can anyone help?

Depending on what you are going to use it for an average altitude may not be very useful.  A bit like saying the average age of a human is 37.

In my case, the average is perfectly OK.

Still, just averaging the minimum and maximum may not give a good representation of where the majority of airplanes fly.  For example in the graph below if we take the minimum to be 8000 and the max to be 39000 the average is 23 but there’s maybe  one point on the graph at 23.

I would assume the minimum operating level is right around 0 AGL.

@tedder42 The answer, and clearly the default interpretation of the question, is cruise altitude.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Different planes with same registration? How is it possible?
I've found a strange thing on fr24: a plane with a B757 icon, but on the photo was a B737.
After a short search I see, both can be true.
There is/was a plane B757, owned by DHL and a B737 owned by Austrian Airlines with registration number OE-LNL
Here is a photo, about the DHL's B757: https://cdn.jetphotos.com/full/6/56676_1648108030.jpg
And another about the Austrian Airlines's B737: https://cdn.jetphotos.com/full/6/18078_1483735375.jpg
How is it possible?

Welcome to aviation.SE! This question is closely related, perhaps a dupe?

The registrations are "recyclable", so when a certain registration is released form a plane that is either removed from service or re-registered to a different country, the previous registration can be used on another plane. This is not optimal if done immediately as it does create confusion sometimes as databases may "lag".
On a further note, sometimes Flightradar24 and other such services simply get these things messed up, showing wrong registration, type and/or picture of plane. This particular case is about re-registration.
You can use planespotters.net to check registration history or aircraft, here is the case for OE-LNL: planespotters.net - OE-LNL. As a perfect case example of the aforementioned database faults, this service has the very first entry wrong (as I'm writing this, database may become corrected later), but both registration entries mentioned in the question are present in this database:
OE-LNL - Boeing 757-200 (current registration at the moment)
Click to embiggen
OE-LNL - Boeing 737-600 (removed from registry July 2012)

In case the database gets updated to remove the out-of-date 737 registration, I've included screen shots of the planespotters.net registration page for both aircraft. Who knows, the 757 may be deregistered and it could be applied to an A350 in the future!

Much appreciated @FreeMan


text:
How much noise does flow separation cause?
I recently found the following image showing the different sources of noise in vehicles. I notice that Aerodynamics is actually quite a big part of vehicle noise, as shown below by the blue line:

Image source
I can imagine this is partly because of the flow going over the car. However, I think a part of this noise also comes from separating flows (for example at the rear window).
I was wondering if it is possible to reduce the aerodynamic noise of vehicles by applying separation control.
As such, I'm wondering:
How much noise does a separating flow generate?
I'm guessing this highly specific to the local geometry and velocity, but I would already be very happy with some examples.

We already have questions about aerodynamic noise so I don't believe this is off topic. This answer seems to address the question at least partly.

I agree this is not off-topic. It is just hard to answer

The Aviation.SE help center lists "Aerodynamics" as a topic, but with the stipulation "(related to aircraft)". This question is clearly related to automotive aerodynamics, with no clear connection to aviation.

Flow separation makes a lot of noise. Usually, it is masked by engine noise and/or damped by insulation. Any pilot of a light plane may climb to a safe altitude and then idle the engine, start a glide, and then slowly pull the stick until the wings start to stall. He will hear a lot of noise, and feel a strong vibration, too... The cause is the turbulence associated with the flow separation.

There are several sources of noise and a complete theory explaining how the noise is created. There are several mechanisms working in a car.
One mechanism appears when you have, as you mention, detached flow. When a flow is detached the flow structures after the detachment point is similar to the following one:

As you can see the air behaves creating swirls. Those swirls interact between each other and generating noise.
So, if you are able to avoid the creation of the swirls you are able as well to reduce the noise generated.
Actually avoiding the creation of swirls and detachment is not the only mechanism, you can always "create different swirls" that generate smaller noise. For example, the antennas are not perfect cilinders and end like a cilinder with an string around in order to generate a different detachment structure, in this case to avoid vibration due to specific tones, this change of structure will also change the noise generate and I think (I can not say for sure) in this case reduces the noise.

In the antenna you show , the "string" is actually the active part of the antenna around a dielectric whip. The helical form is to shorten it. The bottom is a loading coil for resonating the monopole, the tip is a protection. I'm not sure they play an aerodynamic role. Actually this is a common helix-loaded whip or a rubber ducky when the base is not a coil but just the same helix than on the upper part.

It destroys, aerodynamically, the structure of the von karman swirls, breaking them in smaller (and less energetic) swirls. My backgroun is not electronics and communications, I was not aware that as well they had the function. Thanks!

It reminds me of a Megastructures episode where a bridge is subject to rain wind induced vibrations on the cables when there is a certain quantity of water droplets, solved by adding them a helical filet (source). I can't remember which bridge it was.

Is exactly the same phenomena. You made me anxious to see that episode!!!

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
What is the typical drag percentage of air intakes on a commercial airliner
I see several questions on the NACA ducts found on airliners Example 1, Example 2, Example 3 and I was wondering how much drag is normally generated by these inlets?
As they take in air, I imagine there must also be some momentum losses.
I found this source, which gives a nice overview of the distribution of inlet drag for varying locations, but it's hard to judge the total amount from it.

Image source
Is the inlet drag a significant portion of the total drag? What percentage of the total drag is it?

The legendary Fluid Dynamic Drag by Sighard Hoerner contains all the numbers you need. Chapter 9-E should be what you're looking for.


text:
Can fighter jets still fly with their radios being jammed?
Imagine a scenario, a squadron flies through an area and suddenly their radio is getting jammed. Even when they try to go back, it's still getting jammed. Would something bad happen if they can't escape the frequency?

are you confusing radio jamming and EMPs?

What is your definition of ‘bad’ in this scenario?

If you're concerned that the planes would just fall out of the sky because of some radio frequency noise, then no, that won't happen. An EMP (Electromagnetic Pulse) could, conceivably, cause the electronics that control all modern fighter aircraft to fail, however, they're hardened against EMP damage, so that's not guaranteed to happen, and even if it did, it doesn't mean the aircraft will immediately start to tumble out of control.

They wouldn't be very useful if a few jammed frequencies kept them from operating.

There are a bunch of different "radios" on an airplane.

There is a radio for voice communications with other airplanes and Air Traffic Control.
The airplanes can clearly fly without talking to anyone, just an increased risk of getting to close to other planes or not coordinating properly.

There are navigation radios that are Receive-Only, such as VOR, NDB, GPS, and ILS. If the jammer can prevent the signal from being received, these services go out.  These are very handy services to have, but are not essential for safe flight.  Flying without them will make navigation and landing more difficult, but not impossible. (especially if visibility is good).

There are navigation radios that are Transmit and Receive, such as DME and ADSB. These could be jammed either to stop the transmission from the airplane or the reception at the airplane.  Again, losing these services would hinder navigation and awareness, but are not critical to safe flight.

There is sometimes a radio-altimeter that that can determine height above the ground.This is typically only used on Landing, and not critical to flight or landing.

There are probably several other radios that may or may not be present on airplanes, such as ACARS, but none are essential to fighter jets safe operation.

Its not clear what you mean by "something bad", but well trained military pilots would be able to fly,  complete a mission, return and land, using nothing more than a pilot's eyeballs and training. (although with slightly increased risk without all the wonderful technology)

Technically the pilot's eyeballs use "radio" as well... in the terahertz frequency band.

Technically, @randomhead, that can get "jammed" as well. That's why it's a US federal crime to point lasers at planes in flight.

The one most likely to be a problem is the IFF system. If air defences cannot identify the plane as a friend that could result in "something bad happening".

Just to make sure you understand the concept of "signal jamming": simply put it means disturbing the signal such that communication (data transfer) becomes impossible due to noise. It does not mean locking up of the devices using the frequency range being jammed. So a situation where radio jamming is present does not mean a certain frequency cannot be "escaped", it means the frequency range becomes unusable.
Operating fighter jets (or pretty much any aircraft) is not critically dependent on radios being functional. For sure it will complicate things to a certain extent, but there are procedures to follow in case radios fail or frequencies are being jammed.
Fighter "squadron" on a mission has pre planned and practiced scenarios where radio operability is lost. Depending on type of mission, the mission may continue with other means of communication (data link), it may be aborted, or anything in between (for example switch to secondary goals/targets if coordinated strike is no longer possible).
P.S. A fighter squadron is quite a large unit, about 12 to 24 aircraft, it is unlikely a whole squadron would perform a single mission in unison. More likely unit pro perform a coordinated single operation would be "a flight" which is a third or a quarter of a squadron: Wikipedia - Squadron (aviation)

Well when I say squadron I thought it would be like 5 or 4 aircrafts. But what if (just pretend) the mission they're going has some kind of jamming signal on the area but they need to go through since there's something in that area they need to destroy?

In the case of a strike mission, the mission plan  will be such that once the aircraft are in the target area, no communications are necessary. They can carry out the mission without any communications and return to base after that.

Military maxim I learned early: "Comms are the first thing to go."

Absolutely @WilliamWalkerIII  that's why a lot of the military training aims at hight operational capability as a team even with poor or non existent comms.

I don't know how technology in this field has advanced since 1992 or so when I retired, but back at that time, it was not possible (due to power requirements), to jam the entire VHF or UHF radio spectrum from a distance with enough power to inhibit communications between transceivers that are closer together (inverse square law). So Jammers had to detect which frequency or frequencies were in use and concentrate jamming energy on them. This fact allowed creative solutions to the jamming problem (details of which were at that time classified).


text:
Body angle rate and body angular velocity
Are these terms used interchangeably to mean the same things?
I'm aware of the conversion between body rates (often, $[p, q, r])$ and Euler rates (commonly, [$\dot\phi, \dot\theta, \dot\psi])$. This answer suggests that Euler rates are inertial angular rates.
As written in this answer, it would appear that 'body-frame angle rate' is the same as 'body-frame angular velocity'. I expected angular velocity (as a 3-vector, $\vec\omega =[\omega_1, \omega_2, \omega_3]$) to be a different quantity -- referring to wiki for 3D angular velocity.
Is angular velocity (expressed in body frame) the same as angle rate-of-change (expressed in the same body frame)?

In the industry, the elements of the angular velocity vector are called body rates, that is, each element of $\vec{\omega}=[p,q,r]$ is called roll rate, pitch rate and yaw rate.
However, they are decidedly not mathematical rates from the perspective that they are derivatives of some quantities. Unless it's a single axis rotation, you can't straight-up integrate the angular velocity in the hopes of getting something meaningful.
I've had an academic professor telling me a mouthful for calling angular velocity "body rates", but alas, that's the industry norm.

Thanks for this perspective -- this has confused me for a while! Could you clarify if these quantities are numerically the same? That is, if I know the reported angular velocity, $\vec\omega$, of a body, do its three elements really correspond to the rate of change of the three angles? Or is this generally too implementation/sensor specific?

https://academicflight.com/articles/aircraft-attitude-and-euler-angles/

@WesT No, they do not! That's the whole point! The angular velocity is different than the rates of the attitudes.

@JZYL I know; sorry, I should've phrased it differently: I wanted to know if the vector $[p, q, r]$ (body rates reported from a gyro, say) is indeed the angular velocity vector; that is, it numerically obeys the definition of angular velocity (have a direction perpendicular to the plane of rotation and a magnitude corresponding to the speed of rotation). I think I glean from your answer that it does.


text:
Looking for source of Navaids and fixes for an application I am writing
I was able to get a list of airports and runways from the FAA site but cannot seem to find a list of these items.  https://www.openaip.net/navaids?field_openaip_navaid_type_value_many_to_one=2&title=&field_openaip_navaid_id_value=&field_openaip_navaid_country_value_many_to_one=237 has a list but no way to download it.

From the 28-Day NASR Subscription site you can reach the current NASR Subscription page which includes the following downloads:
https://nfdc.faa.gov/webContent/28DaySub/2022-03-24/FIX.zip
https://nfdc.faa.gov/webContent/28DaySub/2022-03-24/NAV.zip
which contain plaintext information about all waypoints and navigational aids in the NAS.
The page also includes a download of the above information in AIXM 5.1 format.
There are also fix/waypoint search functions available on the AIS and NFDC sites. These are not "downloads" per se but you could use GUI scripting to scrape the fix names from the AIS site, for example.
The FAA Data Portal may be helpful, although for aeronautical data it seems to link back to the AIS site listed above.

Does the FAA publish the sectors within each Center?  I've seen plenty of maps that show the borders of the various ARTCC's, but something that describes the sectors within each Center seems harder to find.  Do you happen to know if that's available nationally, or is it a matter of seeing what each individual center happens to make available?  Thanks!

Not sure you intended this comment to go here? But I know that ZBW publishes a Letter to Airmen (or would that be a Letter to Air Missions?) depicting the internal sectors. I'm not sure if any centers do that, but you can try a NOTAM search and see what you come up with.

This is just what I was looking for.  thanks!


text:
How accurate is the inflight location information?
I'm wondering how accurate the location information displayed on the in-flight entertainment system is. As I recall, it always shows quite direct routes, such as this:

However, I'm wondering how accurate this information is. I can image that because of ATC the route will not be as smooth as displayed above. Does it simply show a predetermined route as function of time?
More towards a specific case, would the people onboard the Malaysia Airlines MH370 be able to see the changed course of the aircraft?

My travelling experience is it's accurate. I have watched it closely during taxi, takeoff and approach at my airport, which I am fairly familiar. The little plane rotates to reflect the airplane's true heading, and I can tell which approach we're using.

It's accurate... if installed. As far as I know Malaysia's 777-200ER's don't have the capability, it's only on the 300ER which they don't use.

Recognize that the airplane icon is longer than the entire space between DC and New York.  Obviously, the actual plane is somewhere under the airplane icon, but that still leaves many hundreds of miles of uncertainty.

I know that this picture was not the right picutre to showcase my question (because of the scale), but I could not find any better pictures.

@JonStory MH has IFE on their B772s. I doubt whether there are any 200ER's without seatback IFEs.

Two major inaccuracies I've noticed on some displays are the route of flight and the location of cities. The route of flight is sometimes just a great circle and has no relation to the actual track. And I've seen cities marked hundred of miles from their real locations. But maybe the aircraft position was still completely accurate :-)

@DeltaLima they have IFE but I don't recall it having flight information, only entertainment

@JonStory ah in that sense. That might be true.

The route is actually quite accurate; it is derived from the navigation system of the aircraft. You can determine your departure and arrival routes pretty well from the inflight entertainment system if you are familiar with the routes on the airport you are flying.
In the image in your question you can see a slight course change East of Newfoundland. This is where the aircraft entered its oceanic track.
If the system was operating on MH370, it would have shown the course change as well.
Below you see what the IFE showed on 11 september 2001 when aircraft heading towards the USA were not allowed into US airspace after the attacks.

Source: airliners.net

The position should be fairly accurate, as it is probably taking from some of the airplane's navigation systems. Even if it isn't, the image you posted can't be expected to be accurate to more than +/-100 miles or so, due to the map scale and size of the airplane icon. But at closer zoom levels, you can see the accuracy is much better.
It's certainly possible that the system uses the pre-programmed flight route and estimates the position based on time. But especially on a long flight such as this one, conditions enroute like winds could significantly change the flight time, enough that it would be obvious that the location was wrong.
Airplanes generally will fly as direct as possible, since that would be the route that requires the least time and fuel. However, winds or significant weather may require the plane to change its path.
In the picture you posted, you can see a slight turn to the right over Newfoundland as the plane is entering the North Atlantic Track it is following for this flight.
So if the MH370 plane had this equipment, and it wasn't somehow disabled, then yes, the passengers would have seen the real location of the airplane.
See also another question regarding this display.

I don't believe Malaysia's 777-200ER's have flight information on the in-flight entertainment system. It's only common on the 300ER's

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
How to control torque effects on the ground?
I'm building a Lippisch ground effect vehicle with it's wingtips almost touching the ground, but because of torque effects the left wing actually does touch the ground, so it can't lift off. It even can't go straight.
Adding ailerons isn't an option because this problem has to be solved on te ground (zero speed) and not in the air (because it doesn't get there).
Does someone know something how I can solve this problem?

In what way is torque affecting things?

It is actually the reaction of the spinning propeller and motor who causes this problem

Is a contra-rotating propellor an option?

Where do you have to place it then?

right in front of it. http://en.wikipedia.org/wiki/Contra-rotating_propellers

I have this motor: https://www.hobbyking.com/hobbyking/store/__14398__Turnigy_Park300_Brushless_Outrunner_1380kv.html
How can I manage to make the second propeller spin in the opposite direction of the first one, when I connect this two propellers with the same motor?

Now you're getting into small scale engineering, and out of the scope of the SE site, but I would have thought a bit of gearing would get you where you need to be

can't you put low-friction skids under the wingtips? and maybe even launch off a low-friction surface?

The Lockheed U2 had the same problem (bicycle wheel base and they had to prevent one end being dragged on takeoff).
They solved it by adding "pogo wheels" that self detach on take off. (the ones on the orange sticks.)

(source wikipedia)

The commenters are right: Either use two separate propellers (left and right) which turn in opposite direction, or use a counter-rotating prop.

This picture of a P-38 shows the principle. You can drive both props from the same engine by using driveshafts or belts. The Wright flyer had such an arrangement.

On small airplanes with very big engines a gearbox with concentric driveshafts is driving both propellers in opposite direction.

A simpler way to have contra-rotating props is to put them at opposite ends of an engine pod.
In all cases the prop torque is reduced to zero, or in case of the podded arrangement, almost zero.


text:
Is the sweep angle of an inverted wing zero degrees?
Is the sweep angle of an inverted wing zero degrees, when the angle between the fuselage and the inverted delta wing is 90 degrees? Or is it different because the span of the wing isn't constant over the chord.

The term 'inverted' is normally connected to flying upside down. It would help if you add a sketch - I guess you mean a delta wing with the tip pointing aft.

Sweep is normally given as the quarter-chord sweep. If not, it must be made clear by some qualifier, like leading-edge sweep.
Your previous questions let me suspect that you are into a special version of ground-effect vehicles, the Lippisch aerofoil. The picture below shows the X-114.

The wing is a delta with negative sweep, so it has a straight leading edge and a forward-swept trailing edge. The idea is to have a positive angle of attack and very little space between the trailing edge and the ground for maximum ground effect.
Such a wing has a negative sweep angle. If you take the weighed average of leading ($\varphi_{le}$) and trailing edge sweep angle ($\varphi_{te}$), you get the quarter chord sweep angle ($\varphi_{25}$): $$\varphi_{25} = \frac{\varphi_{te} + 3\cdot\varphi_{le}}{4}$$


text:
How can ATC distinguish planes that are stacked up in a holding pattern from each other?
If I understand holding patterns correctly, several planes stack up vertically, each circulating at its assigned altitude until given permission to exit the pattern and enter an approach for landing.
My question is, don't the vertically stacked airplanes appear to be superimposed on the ATC radar? I am wondering how the ATC can keep them sufficiently separated on the screen so they can track them individually.

It would be really rare for them to all be superimposed, that implies that they are at exactly the same position in the racetrack and traveling at exactly the same speed and turning at exactly the same rate. Radar is also designed to declutter and move labels to free area's of the screen.

Really rare, yes, but it does happen every now and then in congested airspace for two acft, three and more is exponentially rare. This condition rarely will last long, as there is minor variation in speeds, and flying a holding pattern is not dead accurate even by autopilots.

Why would it matter if they can be distinguished from each other?

@MikeBrockington Potential human error on the ATC's part that might cause mid-air collisions if the top plane starts losing altitude, or the bottom plane starts gaining altitude and the ATC doesn't notice.

@nick012000   Well, either ATC notices the change of altitude, or they don't. Surely still doesn't matter which blip is which?

Sometimes we can't.
While having two radar targets presented directly on top of each other is not really an issue (most radar presentation systems lets you move the data label around freely, so you can prevent overlap), the radar system itself can sometimes confuse two targets that are very close. A radar will search for aircraft by sending out a signal and waiting for the aircraft transponder to respond. When two aircraft are very close, they will receive the request from the radar almost at the same time, so their responses will be at the same time as well. The radar will receive the two replies on top of each other, which can result in garbling (see this related question).
In some situations, the radar software can also confuse two close targets, swapping the label between them, so aircraft A is presented to the controller as aircraft B and vice versa. We also sometimes see this close to an airport, where a departing aircraft can "steal" the label from an aircraft on short final, making it look like the departing aircraft is actually the one just about to land.
As a result of these issues, we are not allowed to use normal radar separation to individual aircraft in a holding stack. Instead, radar separation will be provided to a defined holding area, which is a geographical area established around a holding stack, large enough to ensure that anyone holding will remain within. Inside a holding stack, as you correctly noted, only vertical separation is used.
Mind you, most of the time, aircraft in a holding pattern are not flying directly on top of each other. That would require them to enter the holding at exactly the same time, fly at the exact same speed and turn at the exact same rate, which just doesn't happen. So the issues described above are not a problem most of the time, but they are common enough that we have specific procedures to mitigate the risks imposed.

Do you have only Mode A/C radars where you work? Mode-S should prevent all the radar problems you describe.

@DeltaLima We have mode S, but correlation is based on mode A

In that case the radar will interrogate the aircraft one-by-one and the replies will not overlap.

I'd be interested to know if in strong cross-winds (component perpendicular to the inbound holding direction) you find airplanes equipped with only traditional radio-nav equipment flying a different shape of hold compared to the FMC equipped.

Doesn’t ADSB help with the interference issue?

Thanks much for your answer. There is so much to learn in aviation!

Do you work in the US? I think your answer is very specific to a country. In Europe we dont experience such issues and we do indeed seperate to Single aircraft with normal radar separation in a holding stack!

ATC's radar is of the type 'secondary radar' where at the beam does not simply bounce off the object being detected or tracked as it was earlier on the primary radars of old.
On first contact or earlier a 4 digit transponder code is assigned by ATC to each airplane by using the phrase "Squawk xxxx". This code is only valid for that particular flight, or segment of flight.
Secondary radar uses a coded signal from an Interrogator unit which is identified, decoded and responded to by an onboard transponder so that each airplane returns certain data such as callsign, altitude etc. so that each airplane is uniquely identified.
There is also a facility for identifying a blip representing an airplane by pressing an Ident button on the transponder panel in the cockpit which then highlights the blip/symbol of that airplane. This is called "Identing" and is normally initiated by the controller only by saying "Squawk Ident"
Beyond this, the info can be displayed and have many facilities limited only by software processing, the info can be processed and displayed in a customized manner, to make it more useful to the radar controller.


text:
Do additional passengers impact on weight and balance of a domestic airliner?
Recently my kids and I were denied access to our flight because we missed the check in window.  We were informed that new calculations would have had to have been made by the pilot, despite not having had any check in luggage.  When are these calculations made?  Before or after the passengers board?  Apparently, our names did not make the manifest in time, despite nobody having boarded the plane yet.  We were left stranded at the gate.  Can anybody explain the procedures regarding these calculations, because I cannot understand why we were not permitted to fly?

What kind of plane was it?

Thanks for the responses... to put this simply, there were 5 of us in total who were denied the flight.

Thanks for the responses... to put this simply, there were 5 of us in total who were denied the flight. Passenger A - 70kgs, Passenger B - 55kgs, Passenger C - 50kgs, Passenger D - 50kgs, Passenger E - 30kgs.  Total 'carry on baggage' weight - 40kgs.  Flight was Brisbane to Melbourne (2hrs 10mins). Aircraft type Airbus A320.  I guess what i'm trying to determine is: Would there have been a significant alteration to the weight and balance calculations had these 5 passengers been permitted to fly?

Probably not, but if something happened and it was discovered that the Captain allowed people to board AFTER the W&B was done, his ass would be grass.

Also, there may have been manifest issues. On a commercial plane you cannot fly unless your manifest is accurate and you can get in big trouble if you fly with a bad manifest. It is possible the airline's W&B system was tied to the manifest somehow, so if he updates the manifest, he HAS to recompute the W&B, which may require additional time. In this situation, the Captain may not want to change the manifest, just on time and scheduling considerations alone.

Thanks for the responses all...am more interested in the 'long' answer though... this is now in the hands of the relevant ombudsman (Airline Customer Advocate) as I cannot let this rest due to the impact this has had on my family (very long story).  Besides, worst case scenario for weight calculations would have incorporated the higher end of the weight range for each of the other passengers already checked in for the flight, so there would have been plenty of 'slack' already in the total calculated weight to account for my wife and 4 kids.

You're really asking two questions here
1) What is the procedure (ie when are the calculations made)?
2) Do additional passengers affect the weight?
What is the procedure
As soon as check in closes, the numbers for passengers and bags is finalized - nothing else can be added to the plane, otherwise it could potentially run out of fuel. Passengers can be removed, but the captain must be informed in case it affects balance.
At this point the captain calculates the required information for weight, fuel, and balance. It's done shortly after check in closes as there is a limited window in which to add extra fuel etc - remember that the tanker needs to be long clear before the engines can be started and other departure checklists can be run.
This is done so early because the pilot needs time to prepare the aircraft for departure, he doesn't have time to mess about later on adjusting the numbers, and if he has to re-calculate later it adds a whole extra area where mistakes can be made - especially since later on the pilot is likely to be more distracted. Doing it once is fairly easy, you plug the number of passengers into a set of sums along with temperature and other related factors, and it works fuel load out for you. If you try adding passengers you have to re-calculate this, then work out how much extra to add... it's just a whole bunch of extra things that can go wrong.
Do extra passengers affect the weight/balance of the aircraft
Short answer: yes
Take an A319 as an example aircraft: empty, it weighs 40,000 kg, up to a maximum of around 60,000kg
Now I have no idea what you and your family and bags weigh, but let's produce a range. Please don't be insulted by the range: the airline aren't judging your weight when deciding if you can board late: they have to have a single rule regardless of whether it's a skinny teenager travelling alone or a party of 15 obese people

You're an adult, so you presumably weigh anywhere between 50 and 150kg
You said children but don't state an age, so they could be newborns or adults: weight anywhere between 2kg and 150kg
I don't know how many children you have, but for the sake of argument I'll guess it's between 2 and 5 (as you said "children") - that's somewhere between 4 and 750kg

So your total weight for a single family is anywhere between 54kg and 900kg. Chances are most families are more like 250-300kg (a couple of adults at 60-100kg each, and a couple of kids at 20-40kg), but airlines have to assume worst case, otherwise their check in staff have the impossible job of deciding who's too fat to board late.
So take that worst-case scenario (6 obese people) and add a 7kg carry-on bag each, that's nearly 1000kg of weight which the pilot doesn't know on the plane. Or over 2% of the aircraft's empty weight and nearly 1.5% of the maximum takeoff weight. This is assuming they have no hold bags.
Even if your family only weighs 250kg between you and your bags, that's still nearly 0.5% of the aircraft's empty weight - a not insignificant number.
And finally you have to remember that by carrying the extra 0.5% of weight in people, the aircraft needs some extra fuel. And a bit more fuel to carry that extra fuel, and both of those extra amounts of fuel change the balance of the aircraft.
In most "normal" flights you aren't going to make a huge enough difference to balance to cause a crash, or change the fuel requirement enough to cause an issue, but if the aircraft had to divert, you could be the difference between running out of fuel or not, and in some worst-case circumstances a really big family could upset a small aircraft.

I doubt this is as much an issue on a 747 as it would be on a much smaller aircraft.

While a Captain may do the weight and balance on small aircraft, on large aircraft that function is handled by others. When I was flying Metroliners, I did it. When I was flying 747 passenger aircraft, dispatch handled that. When I was flying 747 freighters, the loadmaster assigned to the flight did it.

@terry - that's certainly true in at least some cases: regardless, that person usually has other tasks and the airline doesn't want them having to do it again (usually with trickier sums), wasting time re-stowing and adding fuel etc just for a late running passenger

This is a good answer, but it's missing a operational detail. When the flight plan is issued, the fuel needed is often based on having a completely full passenger load. In addition, each airline allows for LMC (last minute changes) to the final load. As long as the aircraft already has enough fuel for a full passenger load, accepting additional passengers late shouldn't require additional fuel uplift.

The Advisory Circular "AIRCRAFT WEIGHT AND BALANCE CONTROL" (AC120-27E) describes one way in which passenger weights and baggage weights may be calculated by the airlines for various passenger populations, aircraft sizes and configurations, and season (summer or winter, because of the effects of density altitude).
What may have happened is that they had already calculated W&B for the flight, loaded a certain amount of fuel for the passenger and cargo weight, flight plan, and weather, and didn't want to (or couldn't) recalculate.
You didn't say what kind of aircraft you were flying on, but you should know that the smaller or lighter the airplane, the more sensitive it is to weight and balance. When I've flown on a Dash-8, a certain number of passengers had to be fore, abeam, and aft of the wing in order to maintain the center of gravity.  On a larger aircraft such as a 747, there is more flexibility with W&B.

Although your first and second paragraph are spot on, the third is a misnomer - while it would certainly be more possible for an airline to allow passengers to board a larger aircraft late, in practice they all implement at least a "No boarding once the calculations are done" rule regardless of aircraft (and in most cases, they implement a "No boarding when check-in is closed rule) - simply to avoid employees making judgement calls on what counts as a "big enough for late boarding" aircraft

OP wrote: "Apparently, our names did not make the manifest in time, despite nobody having boarded the plane yet." Maybe I interpreted this wrong, but I took it to mean that the they arrived at the gate before anyone had boarded and were denied boarding. As a former Global Services flier on UA, I never had a problem walking up to the gate and getting on any flight, even after the last pax had boarded. So I question "in practice they ALL implement..." (emphasis mine), since I have had first-hand experience with walking on.

"On a 747, they don't really care where you sit." From my experience they might even on sizable plane. I was on pretty empty flight one time and bunch of us were instructed to change our seats after boarding (to the window seats at empty rows). Think it was 737 though.

@Rarst do you know for a fact that you were asked to move because of weight and balance, and not because of some other operation consideration?  It could happen as well on any lightly loaded flight, so a nearly empty 737 on a short hop in good weather could be subject to W&B seat reassignement.

@rbp I am not confident since I wasn't explicitly told the reason, but it looked like they were distributing people along the edges of plane (they did a pass of it, looked, then did another).

@rbp - other than "Several years working in the airline industry" I have no reference for this: however I'd suggest that you arrived at the gate before the numbers were passed to the Captain, suggesting the airline implements a "No extras once the calculations are made" rather than a "No extras once check in officially closes" policy. I've heard of both, but never a "Re-calculate the numbers later" - frankly, no airline wants to waste that much turnaround time and miss losing a runway slot somewhere.

I've clarified and softened the third paragraph

I was in aviation many years in 3 countries in Traffic/Dispatch/Load Control (weight&Balance).
As most say on here the smaller aeroplanes are more balance critical.
I used to take great pride in getting an aeroplane closed up and ready for push back on schedule. I also used to take great pride in getting late pax on board and still getting a schedule. The loadsheet has a box at the bottom labelled LMC. Last Minute Changes.  There is a specific weight allowed, dependent on aircraft, acceptable without having to recalculate. But a competent loadsheet officer should be able to within a very short period.
Flying is not the pleasure it used to be, security has made it far more complex.  I was denied boarding recently in Sofia, check in was just closing as I arrived and the last pax just going through security. The EasyJet aeroplane had not even taxied on to stand and it had a thirty minute turnround. I had no baggage.  Imagine how angry I was, knowing the system, knowing that checking me in would make no difference to the departure. To have a check-in agent telling me the Captain had already completed pre-departure procedures and would not allow a late pax. It was absolute bull, despite how cross I was I knew it was pointless getting angry at check in but internally I was seething.
The next flight to MAN wasn't until the following day so it was either  a hotel or flight to London and then train to MAN.  Of course these cheap airlines do not give refunds or transfer so it was a new ticket at great expense.
There-in lies the crux in my opinion.  I would suggest that the executives at the head of companies such as EasyJet have the policy as a way to increase revenue.  At small airports such as Sofia it takes hardly any time to pass through, security & immigration. Obviously the bigger the airport the more time it takes.  Gone are the days when a breathless Pax Agent would arrive at the gate 10 minutes before push back saying, please can you get these late pax on?

Welcome to Aviation.SE! the first part of this answer sounds useful and would benefit if a bit more details could be added, the rest looks more like a rant and as such is not a good fit here. please consider improving your post.


text:
Service to maintain database of airports, runways, helipads, navaid and fixes
I had asked previously for a list of all the data I mention but now would like to be able to maintain this list.  I know this is not a site for software developers but does anyone know of a site that has an API to check for addition, deletions, or modifications to this data?
I have found some of the data from the FAA that is on ARCGIS but some are missing.

At the bottom of the d-TPP webpage are downloadable ZIP files.  File "E" has a XML file that catalogs changes to the plates in the current update cycle.
https://www.faa.gov/air_traffic/flight_info/aeronav/digital_products/dtpp/

Most of this information is on an update cycle.  Whether changes are made or not, old information expires.  You don’t need a special app to look for changes, just ensure your data is current per the schedule.  There are several subscription providers who support their hardware and software platforms that will prompt you to download correct versions.

Thanks for the reply.  I am actually looking for a source of this information and am going to write my own program to update my database.

Ok, well you specifically asked about maintaining current data, so I assumed you had a source already.

I probably didn't phrase my question very well.  I loaded all the data from the FAA static csv files but now want to keep it updated.  I have contacted Jeppesen to see what services they have.

But can you not write a routine to reperform the download at scheduled intervals?

I want it to be totally automated and run from a web server so scheduled downloads unfortunately would not be an option.

Tell me if I'm being too nosy about something proprietary you hope to profit from, but perhaps if we understood what you are trying to accomplish we might be able to help more.  (i.e. what are you trying to automate?)  And it's hard to know how much you already know, but have you looked into pulling from NOTAMs?


text:
Control surface deflection during takeoff
What is a reasonable (or typical) control surface deflections required for takeoff?
I would like to know two ball park values if possible

elevator deflection for conventional transport aircraft
elevon deflection for aircrafts with no horizontal tail

what's a "conventional transport aircraft"? a Cessna 172, or a B747? what have you searched? what have you found and why was it not satisfactory? how are two random numbers given by internet strangers going to be helpful?

Required for takeoff - essentially none, when an aircraft reaches takeoff speed then it will fly with minimal deflection of the control surfaces provided it’s trimmed correctly for that power and altitude.  Aircraft with ample thrust and/or speed can rotate to achieve a high rate of climb and so a pilot might apply a small control input over a period of time or a radical input for a short period.  The angle of deflection would depend on the physical (or fly-by-wire firmware) linkage between the stick and the elevators, which would again be designed based on the size of the control surfaces and a number of other factors.  To Answer your question directly, the required angle is, or should be, zero.

"provided it’s trimmed correctly for that power and altitude." Well, no, trim also determines (indicated) airspeed. If the plane takes off (because of the angle of attack due to landing gear geometry etc.), but is trimmed for a higher airspeed, it will immediately pitch forward again. Trim is achieved by control surface deflection (in various, more or less direct ways), but of course one can argue what the reference position is against which you would measure the required deflection for takeoff.

When reaching Vr, most pilots make a significant pitch control input to rotate the a/c to the desired pitch attitude.

@Jim they do and this is to achieve a desired rate of climb.  Unless the undercarriage presents the airframe at an angle where flight isn’t achievable, the aircraft will (eventually) get off the ground without rotation

@Frog - Yes, but that is not typical (as OP asks).

I’m reading the ‘required for takeoff’ part, I may have misinterpreted the OPs intent

Transport airplanes are trimmed for takeoff to achieve a speed that is roughly V2 +10-20 kts.  At Vr, it takes a fairly modest elevator input to achieve rotation, depending on how aggressive the pilot wants to be with it, and initially most or all of the elevator input is removed once the nose comes up.  You might pull somewhere around a quarter the available elevator travel, or if you are taking your time, less than that.


text:
Why are conventional gear planes designed with the nose pointing up?
Most of the tricycle gear planes have a level or mostly level posture when on the ground. Why weren't coonvention gear planes designed like this? It would give pilots better visibility and give passengers a more comfortable seating position when on the ground.
A few hypotheses I have gathered are:
Prop clearance, i.e., designers can fit larger propellers on an engine if it was farther away from the ground. However since planes pitch downward slightly when accelerating for takeoff I don't think this could have been a consideration.
Center of gravity. Early engines were heavy and when you mount one on the nose you would want to tilt the plane upwards so it doesn't flip forward due to the weight of the engine. I'm not sure to what extent this is true. And if it were, couldn't they just shift more weight from other parts backwards?
Being able to pitch up for take off. If the plane with a long tail wheel strut tried to pitch up, the tailwheel would be pressed into the ground. In this case couldn't they design some kind of suspension for the tailwheel?
Sorry if this is a silly question. I have absolutely zero knowledge in aircraft design and this just occurred to me as odd.

Tricycle gear airplanes typically "rotate" for takeoff-- the nosewheel is lifted off the ground to place the aircraft in a nose-high pitch attitude, to increase the angle-of-attack of the wing.
If a tailwheel gear airplane were set up to have the fuselage basically horizontal on the ground, it too would need some way to "rotate"-- to lift the nose-- to increase the angle-of-attack of the wing for takeoff.1  But the tailwheel (touching the ground) would prevent this.

Being able to pitch up for take off. If the plane with a long tail
wheel strut tried to pitch up, the tailwheel would be pressed into the
ground. In this case couldn't they design some kind of suspension for
the tailwheel?

Yes that's basically it.  You seem to be imagining some kind of elaborate suspension that would keep the fuselage level most of the time but would allow the pilot to still push the tail down for takeoff?  That seems like a lot of weight to add to the tail of an airplane.  CG issues can always be dealt with by lengthening the nose, but the increased pitch rotational inertia would likely be objectionable.  Plus the added weight and complexity simply wouldn't be worth it.
Actually we could introduce a further nuance by asking "given that tailwheel airplanes typically lift the tail to take off, why not make the tailwheel long enough to support the aircraft in the usual take-off attitude, even if the fuselage is still somewhat nose-high rather than fully horizontal?"
The answer is that 1) there would be some extra weight and drag, and 2) it's advantageous to be able to land the aircraft in a pitch attitude and angle-of-attack where the wing is nearly completely stalled, which is a higher pitch attitude and angle-of-attack than would be safe for a normal takeoff.
Footnotes--

Unless of course the wings were attached to the fuselage at an unusually high "angle of incidence", so that they would be in a good position to create lift for takeoff even with the fuselage horizontal.  This would have other disadvantages-- the fuselage would be flying in a nose-down attitude when the wings were flying at the relatively low angle-of-attack typically used for cruising flight.  Of course there are exceptions to every rule, and there are some aircraft designed to be able to lift off the ground without "rotating" at all, so all the wheels lift off at the same time.  Most (but not all) of these aircraft sit in a rather nose-high attitude on the ground.  Others have minimal fuselages so fuselage drag is low regardless of pitch attitude.  To the best of my knowledge none of these aircraft have either "tricycle" or "tailwheel" landing gear, though many aircraft with "tailwheel" landing gear can be safely "3-pointed" (a take off with all three wheels lifting the ground at the same time) in smooth conditions.  Examples of aircraft designed to normally take off with all wheels lifting at the same time -- B-47, B-52, Aerianne Swift ultralight sailplane.

I imagine some sort of sturdy spring on the tail wheel could keep a aircraft level as well as allow the pilot to pull up no? especially since a majority of the weight is on the main gears anyway. perhaps early designers could be excused for not having adequate technology, but surely we have strong lightweight suspension systems these days? I can see the hindrance to landing at a high angle of attack though. that might necessitate some mechanism to adjust the tailwheel

Principally conventional landing gear was used in the past for simplicity, lighter weight and propeller clearance.  It still remains a popular configuration for aerobatic aircraft, ultralights, and bush aircraft where propeller clearance from rocks and other kinds of debris is needed. It’s also less prone to digging into soft or waterlogged fields than tricycle landing gear is.  The tail wheel design also prevents tail strikes from over rotation during takeoff and landing.
The downside to conventional landing gear is that it’s dynamically unstable, and proned to ground looping if disturbed.  It is also more difficult to control and steer an airplane with conventional landing gear than a tricycle gear configuration.

If it is the nose up attitude that helps with prop clearance, wouldn't that be nullified when accelerating for take off since the plane pitches down to a more level attitude?

@AirCanada.  Not necessarily.  A pilot can perform a three point takeoff if they feel prop clearance is imperative during the takeoff roll.

Note that it is possible to design the AoA of a wing on a tricycle-gear plane in such a way that when you hit the right airspeed, the plane lifts off and flies itself off the ground, after which it proceeds to climb in a more-or-less "flat" attitude. This means the airplane designer has the flexibility to either have the plane break ground without rotation, or stay stuck to the ground until the pilot hits the rotation speed and then pulls back to increase the AoA- after which the plane climbs out.


text:
Why can't supersonic flow work its way upstream?
I am reading this textbook today. And it described supersonic flow as follows:

In a supersonic flow, because the local flow velocity is greater than the speed of sound, disturbances created at some point in the flow cannot work their way upstream (in contrast to subsonic flow)

And I am confused, I don't understand why the flow cannot work their way upstream. I don't really understand what upstream or downstream mean. Can someone explain to me please?

I don't really understand what upstream or downstream mean. Can someone explain to me please?

For an observer sitting in a flying airplane, it looks as if a stream of air is rushing towards and then past him. Therefore, upstream means "in the direction of flight" and downstream means the opposite direction, the perceived direction of travel of the air.

why do fluid particles interact at the speed of sound?

In fluids (matter in the liquid or gaseous state) the molecules are not packed together in a rigid structure (that would be the case for solid matter) but have some space for moving around. The average speed of this movement is equivalent to the temperature of the fluid (more precisely: The square root of the temperature, measured from absolute zero). The average length of undisturbed motion is very small, though, because molecules are constantly bouncing into each other. A local pressure change (say, by an approaching airplane) will cause more bouncing in one direction, because some molecules will be hit and pushed away by this airplane. Now this disturbance will propagate into the fluid at the speed of their movement.

Propagation of disturbances. The circles symbolize the distance a disturbance has travelled, so bigger circles show disturbances which emanated earlier from the airplane.
At rest ("Stillstand"), the disturbances (which we perceive as sound) will propagate equally in all directions. For Mach 0.8, this is no longer true because the source of sound moves while the sound waves propagate away from it. Still, sound manages to move in all directions, so the pressure changes caused by the approaching airplane are "announced" to air upstream of the airplane.
At Mach 1.4, however, the airplane moves faster than the disturbance. It will hit air which is completely unaware of its coming. This is described by "disturbances created at some point in the flow cannot work their way upstream" in your text. The sound caused by the airplane will not reach the air upstream (= ahead) of it before itself does so.
If you want to know what consequences this has, read this answer.

Very good. I would add one (minor) addition or disagreement. What you say also applies identically to solids as well as fluids. Imagine a steel bar flying past you faster than the speed of sound (in steel). Disturbances in that steel bar would only move downstream, not upstream, ( relative to your position), for exactly the same reasons.

Thank you very much! The three examples really cleared up my confusions.

@CharlesBretana Agreed, there is also a speed of sound in solids.

One more scenario where understanding comes from understanding frame translation.

Speed of sound in water is around 1800m/s, in aluminium around 6000m/s off the top of my head.

Because the speed of sound is actually the mean velocity of the fluid particles, that's why it is dependent on temperature. The temperature is really just another way to measure the average kinetic energy (aka velocity) of all the particles in the fluid. A disturbance moves due to the particles interacting with one another. Clearly, if the average free-stream velocity of a body in a fluid is faster than the average velocity of the fluid particles, any disturbance or change in the fluid cannot propagate upstream. The stream is moving faster in the opposite (downstream) direction than the disturbance can move in the upstream direction.

Oh so if I understand correctly, fluid particles interact at the speed of sound, but if the free stream velocity is supersonic, all these particles would get pushed toward the downstream direction. Is that correct? And just one more question, why do fluid particles interact at the speed of sound? Sorry if it's a really dumb question.

@Yihong Zhu it’s not so much that the particles interact at the speed of sound, but the speed of sound is the speed at which particles interact.  Hopefully it’s obvious when stated that way.

@Yihong, Particles interact at the speed of sound because sound is really nothing more than movement of the fluid particles. Sound moves through the interaction of fluid particles, so it of course moves at the speed of the particles.

When something, like our vocal chords, or a speaker cone, pushes on a fluid, (actually, on anything), the particles it pushes directly on move a tiny bit, and push on the particles next to them, and so on,.. and so on...

@ymb1, true, but I felt that getting into this point, albeit technically accurate, would obscure rather than enhance understanding of the physical nature of sound and its relationship to the question posed by the op.  Sometimes simpler is more clear.  But thanks !

Just a short addendum here.
Unfortunately air and waves moving in it are invisible. But luckily in nature waves exist that can be actually easily seen. These are the waves generated by perturbations at the interface between a liquid and a gas: the waves generated by a duck swimming in a lake
When the duck makes some movement at zero speed (maybe it is just moving the neck or stretching the wings), it generates waves on the surface of the water which spread away with a certain constant speed. By a physical and mathematical point of view, exactly the same happens when a body moves in the air: it generates waves that spread away with the speed of sound. This correspond to the left part of this picture, taken from another answer, that I repost here:

Now, when the duck starts to swim, the waves  tend to get closer in front of it and rarefy on the back. The duck "moves toward" the wave front. This correspond to the picture in the middle.
The nicest part is anyway when the duck overcome the "sound barrier" and swim faster than the wave's speed: the waves that it generates are now slower than itself and therefore they lag behind it i.e. they

cannot work their way upstream.

Downstream, they coalesce forming the typical Mach cone.

Beware of the duck approaching at supersonic speed. Source: https://i.pinimg.com/originals/94/d5/7e/94d57e09290b005273aa26313d355443.jpg
If you don't want to wait for some duck, just play with a twig

Actually, the angle of the wake on water is independent of the speed of the object which causes it and is $\text{arctan}(1/\sqrt{8}$) ≈ 19.47° to either side.

@PeterKämpf: the duck appreciate this precision Anyway never spoke about angles in the answer or any relationship with the actual speed... Plus, at zero speed the waves look concentric and not forming any angle, so there it seems to be a relationship with speed.


text:
Temperature profile for a boundary layer
So I was reading the textbook today, and I see this picture for the temperature profile through the boundary layer.
And I was confused on why the shape looks like this?

Should it not look more like this where the temperature is hotter closer to the surface, and gradually cools down as we move away from the surface?

Does "the textbook" (which one?) give any boundary conditions? I would think the temperature profile only looks like this when the surface temperature ($T_w$) is lower than ambient ($T_b$)? The textbook profile looks like it does increase towards the surface (higher speed gradient, so more "friction" and heat production, just like you suggest in the second plot?), but then cooling towards the wall?

@Raketenolli, the textbook I’m referring to is “fundamentals of aerodynamics”. I think the only condition it gave was no slip. I don’t understand why the temperature would cool down close to the wall.

What if the wall is much cooler than the gas?

Look at the positions of $T_w$ and $T_b$ relative to the $y$-axis - $T_w$ is lower than $T_b$.
So the question then, is why does the temperature increase in the middle of the boundary layer in the first diagram and not for the second?
The second diagram is graphing stagnation temperature, not static temperature. When turbulence/viscosity converts kinetic energy to thermal, it increases the static temperature but the stagnation temperature remains constant. (Stagnation temperature is the total of thermal energy + kinetic energy). So the bulge in the first image is because the static temperature increases from turbulence/viscosity converting the kinetic energy, no bulge in the second because it's showing stagnation temperature which already includes the kinetic energy.

Thank you. So the first diagram is static temperature, and it’s zero at the wall because the kinetic energy there is zero, and it increases due to friction. And then it falls due to the cooler air outside of the boundary layer.
Does that sound correct?

It's not zero at the wall, it's equal to $T_w$ at the wall - the distance of the line from the y-axis represents static temperature, and the dot marked $T_w$ isn't on the y-axis. $T_w<T_b$, so a plot of stagnation temperature would decrease continuously towards the wall - looking like the velocity profile with a constant value added.
Because it's static temperature, it increases in the middle of the boundary layer where the conversion of kinetic to thermal energy is strongest

thank you very much. I understand it now.

The wall temperature determines t$_w$ due to heat transfer between wall and the air close to it.
The boundary temperature t$_b$ is simply the temperature of air outside and at the outer limit of the boundary layer. So much is obvious.
In between, friction will heat up the air, and the steeper the speed gradient is, the more friction heat will be set free. This means that the boundary temperature will increase only slowly as one moves into the boundary layer from the outside. Closer to the wall frictional heating will increase and have its peak close to the wall, where cooling from contact to the wall is still small. Directly at the wall this cooling effect from contact with the wall will become dominant, so there is again a steep drop in air temperature.
Only when the wall has been heated up by the boundary layer such that there is no cooling effect, the temperature profile in the bottom graph will occur. This is the case for a surface of little heat capacity and an equilibrium condition with no more heat transfer between air and wall.


text:
Why is Dulles Airport (KIAD) not in any chart supplement?
I was recently listening to the KIAD ground feed, and realized that I had no diagram. When I tried to find one in the northeast chart supplement (which does have the information section), I couldn't. Is this because of the special flight rules around DC?
Thanks in advance.

It’s in there.  Look under Washington, District of Columbia.

As an adjunct to this, does anyone know why KIAD and KDCA are listed under "District of Columbia" instead of Virginia, where they actually are? KCVG is correctly under "Covington, Kentucky"


text:
Why does ADS-B altitude from multiple landed planes vary so much from one plane to another?
When viewing ADS-B data from sites like FlightAware and Flightradar24 (and also from a personal antenna), the landed altitudes vary greatly from one aircraft to another.  What accounts for this variance?
For example, given a runway at 500 ft. elevation above sea level, 3 aircraft landing within a couple of minutes of each other show landed altitudes of 500 ft., 600 ft., and 750 ft. (AMSL)
I should also point out that the ADS-B data points are the same across the tracking websites and personal antenna, and the lat/lon values show landed positions - so there is no variance between the tracking sites.

Just as a reference, the operative FAR is 14 CFR § 91.217 which states that the altitude transmitted must be referenced to 29.92 inhg and that ads-b and mode c must share a data source. So I'm not sure what would account for the discrepancy.

I was going to say perhaps the messages aren't that frequent - but ADSB transmits every second (https://www.faa.gov/about/office_org/headquarters_offices/avs/offices/afx/afs/afs400/afs410/ads-b) ..

The aircraft do not send out altitude when they are on the ground.
On the ground, aircraft send out the surface messages, which combine position, speed and heading.
The airborne message formats combine position and pressure altitude in one message, and speed vector and geometric altitude in another.
The lowest altitude you see is the last altitude decoded by the receiver before the aircraft touched down. Unless the receiver has very good line of sight to the final approach and touchdown area, and is not suffering from garbling (overlapping of messages received from various aircraft at the same time), the last altitude you will may be actually be quite high above the runway.

However the planes do transmit their position while on ground, so shouldn't the altitude also be receivable all the way untill the system shutoff? In case the altitude transmission stops with weight on wheels event, last transmission should be very close to field elevation (ambient pressure taken in account?).

@Jpe61 the transmission of altitude seizes with WoW. The last altitude transmitted is typically at or 25ft above the field (after applying QNH correction). But transmitted does not equal received. At low altitude, there is often not a direct line of sight between the transmitting antenna and the receiver antenna. Also, messages get lost due to collision with other messages being received at the same time.

But how does the ground position keep updating?

I've seen "0" on FlightRadar24 for some recently-landed planes where field elevation is approx 1300' -- ?

@Jpe61 Ground position keeps update through multipath (reflections), diffraction etc. but not at a rate of 1/seconds. Flight radar 24 is very good at connecting the dots with lines, but really on many airports the surface tracks are not that frequently updated. 10 second between updates would already result in at >100ft error at 700 fpm descent rate, but is barely visible in the ground track.

@quietflyer pressure altitude can be quite different from actual AMSL value if  QNH is far from 1013.25 / 29.92

@Jpe61 to substantiate the comment about multipath and diffraction: for a long time I operated and ADS-B receiver in an apartment in a city centre. The antenna was mounted outside a window. I could receive ground positions form the local airport at a few very specific locations: these were reflections from a single high rise building about 2km away in the general direct of the airport. At certain weather conditions (strong inversion layer) I could receive messages from the surface of a big international airport 50 km away with a probability of about 1/100 per transmitted message.

Thanks @DeltaLima that makes sense

I often receive ground messages on my receiver, and also many that are not, so these other altitudes must be just prior to landing. That makes sense, but it's still strange to see the variance of a couple hundred feet in similar positions.  But is my understanding correct that all ADS-B out is calibrated to 29.92 and thus all systems "should" report similar altitudes "if" we could see them all at the same exact position spatially?

Also don't forget the ADSB receivers aren't located perfectly, so it's likely planes are going out of reception before landing.

Aircraft DO transmit altitude while on the ground... look at the ADSB data that comes from the aircraft.  If the data hasn't been edited, in the "alt" column you can see clearly altitude IS being transmitted even though the aircraft is on the ground and displaying GND status. The data can also include the corrected altitude and the pilot entered correction pressure that is supposed to yield the correct height above sea level. That resultant varies quite a bit also.
As to why there are altitude variations among aircraft simultaneously on the ground at the same time... varying typically about 200 feet, boggles the mind. It's hard to imagine that a digitally monitored analogue device that is supposed to be calibrated (once a year?), could get so out of whack. But, there's another contributing factor.
Older and less expensive transponders output altitude data in 100 foot increments (newer are 25 ft and a few at 10 ft resolution); this means that when the first pulse of 100 ft altitude appears, you are probably only about 51 feet when the transponder transmits the message.  This step resolution transmission of altitude is usually smoothed by some method of averaging. But what do you do when an aircraft has the right altimeter setting but displays -100 feet when you're at 16 feet above sea level?  And then you move to another aircraft on the ground 16 ft above sea level and it too has the correct altimeter setting but it reads 50 feet?
For municipal and County owned airports, they may opt to buy a service like WebTrak from Envirosuite so aircraft can be accurately tracked locally.  But, in our area, Envirosuite/WebTrak solves the above problem by wiping out all ground data and substituting one of two values that correspond with the elevation of the airport (Geodetic or GPS).  But one sin leads to another.
This clean up means that altitude data has to be 'adjusted' all the way up to about 800 feet. This process erases all the altitude differences of aircraft on the ground.  I'm currently looking for data on normal variations of altimeters.  I guess we could blame it on Cro-Magnon technology of pressure cans moving a spring, but what are the alternatives?  Radar altimeters are very expensive.  GPS has satellite time delays and depends on ellipsoids and Geoids, none of which are universally accepted.  That still leaves us without a satisfying explanation.


text:
How to physically write (log) a three leg xc in Gleim logbook
I just did my solo long xc, over 150nm, from one airport, to another, to a third, and then back to home airport. My question is how to put all this in the entries section, do I put all three squeezed together in the "TO" block, which would be my preferred method, or do I split it into multiple lines. Which is the legal and correct way? Can't ask my instructor since he is on vacation this week. Thanks to anyone who can help.

Did you land at each airport, or just route over them?

Related: https://aviation.stackexchange.com/q/55825/34686, https://aviation.stackexchange.com/a/55829/34686

The simplest approach is to log each leg as a separate flight, and that’s completely legal.
However, my personal opinion is that this wastes space for multi-leg flights.  Instead, I use a single entry and write all the points of takeoff/landing across the FROM and TO columns, ignoring the divider between the columns.
The regulations arguably say that each time you stop the engine, that ends your “flight”. And that rule is nearly as simple to abide by yet still improves over an entry per leg.
If you do decide to combine legs, the key is to never get yourself into a position where you can forget to log one leg if the next leg gets delayed or canceled for some reason—and the odds of forgetting go up the longer you wait to log it.

@757toga We can log all time that we’re “operating an aircraft for the purpose of flight”, e.g. Hobbs time.

@757toga I’ve had legs that crossed into night while I was airborne, so I don’t see how anything changes if it coincidentally happened between legs, or whether I stopped the engine for a few minutes. It’s still all one flight in my mind; just get the times in each column right.

@757toga Thats a decent argument for separate lines if the engine stops, and it’s certainly a simpler rule to explain, so I’ll edit that into my answer.

@757toga, I disagree with that last comment.  If you fly 6 legs in a day, encounter 10 minutes of actual IMC on each leg, you can log it all on one line and take a 1.0 in the actual instrument column.  Nobody cares which leg(s) it happened on, you are logging time, not sorties.

It isn't a matter of agreeing or disagreeing, my point is the inconsistent application of your interpretation.  IF you stated that one should log each leg on a separate line I might not agree, but you would be consistent from the standpoint that total flight times, as well as flight condition time, would be discretely applied to each line.  However, if your position is that you can combine all legs for a day onto a single line with one total for the day, but you must break out subcategories of flight time I would say you are inconsistent, and I don't see a basis for that in the CFR...

Each flight should go on its own separate line, if you fly from A to B then A is from and B is to. If you fly 4 legs, A->B, B->C, C->D and D->E, landing at each then you'd have 4 lines in your logbook. If you fly from A to E, but pass by B, C and D on the way without landing then you'd have only one entry in your logbook from A to E, and you could say via B, C and D in notes.
Note that there's no legal requirement to do it this way, at least in the regulatory frameworks I'm familiar with, however as you are in training you want to be completely clear on the details so you can demonstrate your flights and times to an examiner, the FAA or anyone else who needs to see it. They will expect to see a separate line for each flight, so it's best to give it to them.

It's common among the instructors and examiners I've worked with @757toga. I imagine it's less of an issue once you have enough hours.

@GdD My CFIs always wrote one entry per lesson/flight regardless of how many legs, and I followed suit for my own logging; my DPEs never commented on it.


text:
What does "mode-four check" mean?
I heard this line in Top-Gun (1986):

Voodoo 1: Mustang, when you get the chance, can you give me a
mode-four check?

What does "mode-four check" mean?

Mode 4 is one of several modes of the IFF, (Interrogator, Friend of Foe) system used in military aircraft.
Think of it like a radar transponder, or ADSB-out:  A discrete code, known to other cooperating entities, is entered into the IFF control head.  Other aircraft including AWACS, ship based weapons systems, and fighter aircraft have the ability to transmit an interrogation signal.  If the targeted aircraft is properly coded it returns a signal that identifies them as part of the friendly force.
Once airborne and outbound on a mission it is standard practice to ensure that the system is working before leaving the vicinity of the ship lest you be unable to be identified upon your return.

To add to this, mode 4 is protected by crypto. The interrogator encodes the pulses with special timing, and a transponder that doesn't have matching crypto cannot send response pulses at the proper times - these pulses (at least in the past) would show up as a "fringe" behind the dot on the radar scope, but if the transponder has the right crypto, this "fringe" becomes a nice solid arc behind the contact. It's handled differently these days I'm sure, but the basic idea is the same - it's all about that pulse timing.

@Steve, Thanks for that additional info.  I didn't think the OP was looking to go too deep into details of its operation, and I am only familiar with how it's used in the field.

One (trivial) addition, is that the crypto code is "injected" into each aircraft's systems (both the radar - for request transmissions, and the IFF transponder for responses to those requests), on a coordinated regular schedule. When I was on active duty, that was once every 24 hours.

You can learn more about Mode 4 by reading about IFF Mark XII development.

I can sense when someone asks a question because they are an aspiring avionics technician or electrical engineer, vs when they are a movie fan who just wants to know what something means.  My money for this question on the latter.  I appreciate the feedback, but don't plan to modify my answer to add further detail on the inner workings of the black box.